An optical system has been developed to measure the small motions of a person's head during an MRI scan in order to correct the scan and thereby improve the resolution of the image. The system uses two position-sensitive detectors to sense changes sequentially in the positions of three blinking miniature lamps that are mounted on a fixture attached to a bite plate. The computer-controlled instrument computes the six degrees of freedom (three angles and three positions) that are necessary to characterize head movement.